There is a lot of info on Defender TDCi (puma) driveline clunk clunk, do a search it has all been covered before, but to summarise ....

clunk clunk:
*Free play in splines on rear axles and drive flange, poor quality material, poor spline match (FIX: replace with Ashcroft or HTE (brand names) axles / drive flange)
*Poor preload on centre diff assembly from factory (FIX: rebuild centre diff correctly).
*Wear in adapter shaft splines between Ford gearbox and Land Rover transfer case, assembled dry from factory (FIX: replace and lube splines on assembly, can fix a Ashcroft adapter shaft kit)
*(2.4lt) failing clutch assembly, common issue on other Fords with this clutch (FIX: replace clutch assembly, AP brand kit seem good)
*Poor driving style (FIX: release throttle pedal first, then depress the clutch)

If clutch is good, I say to customers to start at the axles / drive flange first and work your way towards the clutch, if clutch needs doing then start at that end...

A couple of vids on the axle spline free play....
https://www.youtube.com/channel/UC0g...Uwpq2Qivaj_wbg

Regards
Daz